This invention relates to an improved process for the conversion of aromatic hydrocarbons. More specifically, the invention concerns disproportionation and transalkylation of aromatic hydrocarbons to obtain xylenes.
The xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important of the xylene isomers is paraxylene, the principal feedstock for polyester which continues to enjoy a high growth rate from large base demand. Orthoxylene is used to produce phthalic anhydride, which has high-volume but mature markets. Metaxylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less-desirable component of C.sub.8 aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Most commonly, toluene is dealkylated to produce benzene or disproportionated to yield benzene and C.sub.8 aromatics from which the individual xylene isomers are recovered. More recently, processes have been introduced to disproportionate toluene selectively to obtain higher-than-equilibrium yields of paraxylene.
A current objective of many aromatics complexes is to increase the yield of xylenes and to de-emphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. Benzene produced from disproportionation processes often is not sufficiently pure to be competitive in the market. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C.sub.9 aromatics along with toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,097,543 (Haag et al.) teaches toluene disproportionation for the selective production of paraxylene using a zeolite having a silica/alumina ratio of at least 12 and a constraint index of 1 to 12, which zeolite has undergone controlled precoking. The zeolite may be ion-exchanged with a variety of elements from Group IB to VIII, and composited with a variety of clays and other porous matrix materials.
U.S. Pat. No. 4,276,437 (Chu) teaches transalkylation and disproportionation of alkylaromatics to yield predominantly the 1,4-alkylaromatic isomer using a zeolite which has been modified by treatment with a compound of a Group IIIB element. The catalyst optionally contains phosphorus, and it is contemplated that the Group IIIB metal is present in the oxidized state.
U.S. Pat. No. 4,922,055 (Chu) teaches toluene disproportionation using a zeolite, preferably ZSM-5, containing framework gallium, shown to be superior to non-framework gallium. Selective production of paraxylene is not disclosed in this reference.
The accepted mechanism for transalkylation and disproportionation is believed to be effected via a strong Bronsted acid such as is provided by a zeolitic aluminosilicate. A lower-energy path, however, would provide potential for greater selectivity and improved economics.